Among radio transmission technologies, an amplification circuit called “power amplifier” has been known as a device for transmitting power to an antenna. For this amplification circuit, it is required to comply with radio standards determined by respective countries, and further, under such a condition, miniaturizing a transmitting device, lowering costs, and reducing power consumption are cited as important issues.
A major radio standard concerning the power amplifier requires that the power spectrum outputted from an antenna be a prescribed value or less. That is, when a carrier frequency is f0, harmonic components such as multiplying of the carrier frequency, i.e., 2f0, 3f0, . . . , are required to be attenuated to a value according to the radio standard.
As a typical power amplifier, there is one configured with a bias voltage generation circuit 1701, an inductor 1702, an NMOS transistor M171, and a band transmitting filter 1714, as shown in FIG. 33, for example. This power amplifier converts a bias voltage having a sinusoidal waveform properly by the bias voltage generation circuit, and inputs the converted waveform to a gate of the NMOS transistor M171. This NMOS transistor M171 operates as an amplification circuit, and the inductor 1702 operates as a constant power source for alternating-current. Consequently, the inputted sinusoidal wave is amplified and outputted from the amplifier output.
Further, by passing the signal through the band transmission filter 1714, the harmonic component contained in the output signal are removed, so that the harmonic component can be attenuated to the value according to the radio standard, and the sinusoidal wave signal obtained as described is outputted from an antenna 1716 as shown in FIG. 34.
However, there has been such disadvantages that the amplification circuit requires an analog circuit or the like for generating a sinusoidal wave, and its design becomes complicated, and further, since the analog circuit is difficult to be operated by a low-voltage power source, it is hard to be applied to a fine circuit processing whose breakdown voltage is low.
Also, for a power amplifier that does not need to generate the sinusoidal wave, a circuit which uses a pulse wave instead of the sinusoidal wave for input, as shown in FIG. 35, has been widely used. The circuit shown in FIG. 35 is configured with an NMOS transistor M221, an inductor 2202, and a band filter 2214.
However, in this kind of configuration, a waveform of an output 2201 of the power amplifier becomes a distorted waveform which is not the sinusoidal wave. A spectrum of the output of the power amplifier is shown in FIG. 24. There, in order for the signal outputted from the antenna to satisfy the radio standard, harmonic components 2f0, 3f0 . . . are required to be attenuated, and for this, harmonic components have to be removed by using a band transmission filter 2214 with a good harmonic component reduction performance. Further, in order to enhance the harmonic component reduction performance of the band transmission filter 2214, an LC filter having higher order (or using an element having a higher Q value) is required, and this leads to an increase in the number of parts, which has been a cause of increase in mounted area and cost.
Aside from this, as a power amplifier which can realize a low voltage operation without using an analog circuit and a passive element for the amplifier, there is an inverter type power amplifier configured with an NMOS transistor M242 and a PMOS transistor M241 as shown in FIG. 37.
In a case of this configuration, a waveform of the amplifier output 2401 becomes close to a pulse wave. Since the frequency components contained in the pulse wave are mainly a carrier frequency and odd harmonic component, it shows a frequency spectrum of the amplifier output signal as shown in FIG. 38, so there is such an effect that an even harmonic component is suppressed compared to the case of the configuration shown in FIG. 36. However, in the case of FIG. 38 described above, odd harmonic component is not suppressed, so there has been such a problem that a band transmission filter 2414 with a good harmonic component reduction performance is always required, as is the case with the configuration shown in FIG. 35.
As a cause of a harmonic distortion, a nonlinearity is cited. With this, transistor characteristics (gain, series resistance, and the like) are varied according to amplitudes of an input signal and an output signal. As a measure against this, an amplifier parallelization technique, with which a plurality of amplifiers are selectively operated, and when a required output amplitude is changed, it is responded without changing an input amplitude so as not to generate a harmonic distortion, has been known. For example, Japanese Patent Application Laid-Open No. 62-217708 (Patent Document 1) describes a technique which can obtain an output signal with low distortion when a large output amplitude is required, not by increasing input amplitude of each amplifier, but by increasing the number of amplifiers to be operated from among the amplifiers connected in parallel instead of increasing input amplitude.
Further, a constant amplitude wave synthesizing type amplifier of LINC (LINEAR AMPLIFICATION WITH NONLINEAR COMPONENTS) type disclosed in IEEE, COM-22, P1942 (Non-Patent Document 1), Japanese Patent Application Laid-Open No. 1-284106 (Patent Document 2), and the like, employs a method with which two power amplifiers 2601 and 2602 having the same size are connected in parallel, and phases S1 and S2 of input signals 2603 and 2604 inputted to them are changed and added, so as to change the output amplitude of the synthesized signal.
In this case, the amplitude is maximized when the phases S1 and 52 of the input signals 2603 and 2604 are equal, and this time, the amplitude of the output becomes twice the output amplitude of each power amplifier. Meanwhile, when the phases S1 and S2 are shifted by 180 degrees from each other, a sum of the output amplitudes becomes zero. Therefore, the output signal with low distortion can be obtained without increasing input amplitude of each amplifier with this method.
However, since the techniques used in these methods are specialized to suppress a sharp increase in harmonic distortion generated when the output amplitude is increased, it is not possible to suppress harmonic distortion according to the power amplifier itself, which is generated regardless of the output amplitude, such as a distortion due to odd harmonics in the waveform of the output from the power amplifier at the time of inputting the pulse wave described above. Therefore, in order to suppress such harmonics, the band transmission filter with a good performance has been still required.    Patent Document 1: Japanese Patent Application Laid-Open No. 62-217708    Patent Document 2: Japanese Patent Application Laid-Open No. 1-284106    Non-Patent Document 1: IEEE, COM-22, P1942